The present invention relates to acoustic apparatus and methods for fluid flow measurement. More specifically, it relates to apparatus and methods for compensating for variations in the internal diameters of pipes in which flow is measured, for measuring flow in the presence of flow profile variations, for optimizing the location and orientation of a flow sensor and for detecting accumulation of pipe wall deposits.
Insertion probes for detecting the flow of fluids are typically mounted in round pipes having internal diameters that are not precisely known. Because these probes are generally used to measure flow rate by sampling a small portion of the flow profile and deriving from that measurement the volumetric flow rate based upon an assumed internal pipe diameter, a pipe diameter different from that assumed can introduce significant error into the derived results.
The magnitude of the problem can be seen by considering the ASTM A 106 dimensional limits for a range of diameters about a nominal diameter of six inches for Schedule 40 steel pipe. The tolerances are + 1/16″, − 1/32″ in diameter and +15%, −12.5% in wall thickness. The corresponding variation in wetted cross sectional area approaches 2% and provides that degree of uncertainty in determining volumetric flow rate. Those skilled in the art will recognize that tolerances vary with the pipe size and may increase or decrease from the recited example.
The potential error related to this variation in internal diameter has not received much attention in the past because of the acceptance of the probe type of sensing instrument as one of relatively low accuracy and because of the difficulty in measuring the internal diameter of installed pipes. There is, however, an increasing demand for improved performance, which indicates that a practical means of compensation for diameter variations is desired.
Insertion probes generally need to be inserted to a correct depth and to have a precise angular orientation with respect to the direction of the flowing fluid in order to minimize flow measurement error. Feldman et al., in U.S. Pat. No. 6,584,860, the disclosure of which is herein incorporated by reference, teach methods of and apparatus for measuring a distance between a portion of the piping apparatus into which a probe is inserted, as well as for combining the results of these measurements with a presumed pipe diameter in order to insert the probe to the correct depth.
Accumulation of scale and other deposits on the inside walls of pipes through which fluids flow can significantly change the effective diameter of the pipes and can also inhibit flow near the pipe walls so as to cause a change in the flow profile. This is a recognized problem that is not solved by the prior art, which ignores diameter changes and uses a single, preset diameter throughout the service life of a given flow sensor.
Acoustic flow sensing probes using time-of-flight technologies as exemplified in my U.S. Pat. Nos. 6,370,963, 6,422,093, 6,457,371, 6,508,134, 6,178,827 and 6,575,044, the disclosures of which are herein included by reference, teach means for measuring fluid flow rates which are compatible with the present invention.